A lanthanide luminescent cation-exchange material derived from a flexible tri-carboxylic acid 2,6- bis(1,2,3-triazol-4-yl)pyridine (btp) tecton Eoin P. McCarney, a* Chris S. Hawes, a Jonathan A. Kitchen, b Kevin Byrne, c Wolfgang Schmitt c and Thorfinnur Gunnlaugsson a * a School of Chemistry and Trinity Biomedical Sciences Institute (TBSI), Trinity College Dublin, The University of Dublin, Dublin 2, Ireland, D02 R590. b Chemistry, University of Southampton, Southampton, SO17 1BJ, U.K c School of Chemistry and Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, The University of Dublin, Dublin 2, Ireland, D02 XR15. Abstract The synthesis of the three dimensional metal-organic framework material, [Zn 7 L 6 ]·(H 2 NMe 2 ) 4 ·(H 2 O) 45 (1), derived from a flexible tri-carboxylic acid 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) ligand, is presented. The btp ligand, H 3 L, adopts a three- dimensional hydrogen bonding network in the crystalline state through a combination of carboxylic acid dimer and syn-anti- btp:carboxylic acid hydrogen bonding synthons. The Zn(II) species 1 exhibits a three dimensional framework structure with the rare crs topology, and contains linear and undulated solvent channels extending in three dimensions. The guest exchange and gas adsorption properties of 1 were investigated; herein we demonstrate the exchange of dimethylammonium cations from the as- synthesized material with cationic guest molecules in the form of dyes and luminescent Ln(III) ions. Sensitization of Eu(III) and Tb(III) inside the porous network of 1 was achieved upon cation exchange, with a view towards developing functional luminescent materials. Graphical Abstract 1
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A lanthanide luminescent cation-exchange material derived from a flexible tri-carboxylic acid 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) tecton
Eoin P. McCarney,a* Chris S. Hawes,a Jonathan A. Kitchen,b Kevin Byrne,c Wolfgang Schmittc and Thorfinnur Gunnlaugssona*
aSchool of Chemistry and Trinity Biomedical Sciences Institute (TBSI), Trinity College Dublin, The University of Dublin, Dublin 2, Ireland, D02 R590. bChemistry, University of Southampton, Southampton, SO17 1BJ, U.KcSchool of Chemistry and Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, The University of Dublin, Dublin 2, Ireland, D02 XR15.AbstractThe synthesis of the three dimensional metal-organic framework material, [Zn7L6]·(H2NMe2)4·(H2O)45 (1), derived from a flexible tri-carboxylic acid 2,6-bis(1,2,3-triazol-4-yl)pyridine (btp) ligand, is presented. The btp ligand, H3L, adopts a three-dimensional hydrogen bonding network in the crystalline state through a combination of carboxylic acid dimer and syn-anti-btp:carboxylic acid hydrogen bonding synthons. The Zn(II) species 1 exhibits a three dimensional framework structure with the rare crs topology, and contains linear and undulated solvent channels extending in three dimensions. The guest exchange and gas adsorption properties of 1 were investigated; herein we demonstrate the exchange of dimethylammonium cations from the as-synthesized material with cationic guest molecules in the form of dyes and luminescent Ln(III) ions. Sensitization of Eu(III) and Tb(III) inside the porous network of 1 was achieved upon cation exchange, with a view towards developing functional luminescent materials. Graphical Abstract
SynopsisStructural analysis of a three dimensional metal-organic framework material, derived from a flexible tri-carboxylic acid 2,6-bis(1,2,3-triazol-4-yl)pyridine ligand, capable of rapid and reversible guest exchange of various cations and sensitization of lanthanide ions.
environment as a result of the btp syn-syn chelation and the binding of two monodentate
carboxylate oxygen atoms on the benzyl arms of two adjacent ligands.
Figure 2. Fragment view of the crystal structure of 1 showing the connectivity of the two unique
Zn(II) coordination environments.
10
The geometry of Zn1 shows some deviation from that of an idealized trigonal bipyramid with a
value of τ5 = 0.72.32 The basal plane of the coordination polyhedron consists of the pyridyl
nitrogen atom N4 and the pair of monodentate carboxylate oxygen atoms, O1 and O5, whereas
the apical sites are occupied by the two proximal triazolyl nitrogen atoms, N3 and N5. The bite
angles (∠ (N3-Zn1-N4)) and (∠ (N4-Zn1-N5)) are 76.6(2)° and 74.4(2)°, respectively. The N4-
Zn1 bond length is 2.081(6) Å and the N-Zn distances are 2.158(6) Å and 2.206(6) Å for triazole
nitrogen atoms N3 and N5, respectively. The O1-Zn1 and O5-Zn1 bond lengths are 1.924(4) Å
and 1.937(4) Å, respectively. The second unique Zn(II) coordination environment has octahedral
character and is coordinated by six monodentate carboxylate groups. This coordination
environment is particularly unusual; crystallographically characterized examples of fully
monodentate [Zn(RCOO)6] centers have until now only been reported in homo- or hetero-
metallic clusters featuring μ2-κO;κO′ bridging to additional metal ions. This bridging is usually
brought about by the formal 4- charge of the central coordination sphere. However, in the present
case, the non-coordinating oxygen atoms are involved in C-H···O hydrogen bonding interactions
with the nearby triazole C-H groups (dC···O 3.003(9) Å, ∠ (C-H···O) is 149.1(4)°), negating their
ability to coordinate to an additional cation. The two C-O bonds are the same length within error
consistent with a fully ionic carboxylate rather than the more localized bonding mode observed
in Zn1. This highly Lewis basic site is the most likely candidate for hydrogen bonding
interactions with the intractably disordered dimethylammonium cations. A similar hydrogen
bonding interaction also occurs involving the non-coordinating oxygen atoms from the Zn1
coordination sphere, and is common for btp and other 1,2,3-triazole containing species. The
octahedral Zn2 node links six btp ligands, each coordinating through the 4-pyridyl carboxylate
11
position. All six metal-ligand bonds are equivalent (the O1-Zn2 distance is 2.087(6) Å), as the
Zn2 atom resides coincident with crystallographic improper rotation axes.
The intricate connectivity of btp and zinc ions within complex 1 makes a direct comparison of
the topology to a known 3-dimensional net challenging. Avoiding the non-trivial trinodal 3,4,6-
connected network description arrived at by assigning nodes to each unique metal and ligand, the
most sensible topological description was arrived at by assigning a six-connected node to the
central octahedral zinc ion Zn2 which further encompasses all six of the coordinated ligand
molecules and their btp-chelated Zn1 ions (Figure 3). With this description, the network ‘links’
are simply the Zn-carboxylate bonds involving Zn1, and each of these nodes as defined above
links to only six others. Any two adjacent nodes are linked to each other by 4 Zn-carboxylate
links occurring through the pair of symmetry related Zn1-O1 and Zn1-O5 bonds (Figure S3,
Supporting Information). The resulting network topology, although somewhat esoteric, is
described by the crs (cristobalite) net,25 equivalent to the augmented dia-e net. The six-
connecting nodes result in a net
12
Figure 3. (left) Graphical representation of the idealized crs topology.25 (right) The fragment
view of the six connecting node represented by the red spheres in the crs topological diagram.
of edge-sharing truncated tetrahedra ([34.66], purple) and smaller tetrahedra ([34], green).33 No
interpenetration was evident in the structure of 1.
The crystal packing is dominated by rhomboidal cages connected face-to-face to four equivalent
cages by the octahedral Zn2 centres (Figure S4). The remaining two faces, not occupied by Zn2
octahedral sites, are directed into the large solvent channels, which exist in all three dimensions
(Figure 4).
Figure 4. (left) Perspective view of the solvent channels in the crystal structure of 1, hydrogen
atoms omitted for clarity. (right) Perspective view of the solvent accessible surface (blue)
highlighting the channels that exist in a single unit cell.
A mixture of straight channels and undulated channels are interconnected; the three dimensional
network of pores, in total, accounting for approximately 45% of the total unit cell volume. It
should be noted that, taking into account the dimethylammonium cations (64 of which per unit
13
cell, which account for only ca. 2.5% of the total unit cell volume), the volume of these channels
still allows for considerable solvent-accessible volume. These channels are irregular hexagonal
in nature and alternate between two distinct sizes i.e. approximately 21 Å and 14 Å at their
maximum edge-to-edge interatomic distance, respectively (Figure S5, Supporting Information).
The aromatic rings of the ligand benzoate arms dominate the make-up of the edges of the two
differing hexagonal channels with the Zn1 centers at each of the six corners. Following the
elucidation of the channel system in the structure of 1, we turned our attention to guest exchange
studies.
Thermal and gas adsorption studies
Thermogravimetric analysis (TGA) of freshly isolated 1 revealed a multistep desolvation profile
upon heating in a nitrogen atmosphere (Figure S6, Supporting Information). The freshly
prepared, predominantly DMF-filled compound experienced a loss in mass between
approximately 60 °C and 120 °C which then flattened at 160 °C between 65% and 61% residual
mass before another sharp decrease in mass occurred at 350 °C due to decomposition. The total
loss in mass was approximately 39% (wt.) in the 40→350 °C temperature range. After soaking 1
in MeCN, a rapid loss in mass was observed beginning at room temperature and reaching a
plateau at approximately 73% residual mass below 100 °C (Figure S7, Supporting Information).
From this data it can be concluded that the lattice solvent molecules in 1 are readily exchanged
for the more volatile MeCN upon soaking.
The effect of exposing a sample of 1 to the atmosphere was also investigated using
thermogravimetric analysis. The thermal profiles of samples of 1 which had been air dried over
the course of four months were strikingly similar (Figure S7, Supporting Information) and in
14
contrast to a freshly prepared sample comprised predominantly of DMF, as observed by 1H NMR
of the digested fresh sample (Figure S13, Supporting Information). There was a more substantial
loss in mass for the freshly prepared sample, but a lesser loss in mass for samples which were air
dried indicative of the channel solvent being exchanged with water in the atmosphere. This
observation explains why the thermal profiles of 1, regardless of initial solvent occupying the
channels (DMF or MeCN), behave similarly after exposure to the atmosphere for an extended
period. An intermediate stage of solvent exchange with atmospheric water shows a lesser extent
of volatile mass loss (Figure S7, Supporting Information) due to the shorter four week period of
time it was in contact with the atmosphere. This observation that DMF solvent molecules had
been exchanged with water molecules was corroborated after elemental analysis on an air-dried
sample indicated the presence of water within the channels and could not be suitably fitted
allowing to DMF within the channels.
Based on the substantial capacity of 1 for neutral guest molecules, gas adsorption experiments
were also undertaken. Following CH3CN exchange, the sample was activated at 100 °C under
dynamic vacuum overnight. Surprisingly, the CO2 (273K), H2 (77K) and N2 (77K) adsorption
isotherms showed much lower than expected uptake for the thermally activated material (Figure
S8-10, Supporting Information), with a maximum loading of ~ 3.75 wt% CO2 at 1 atm and 278K,
and lower loadings for N2 and H2. These data suggest that the large and interconnected channels
in the as-synthesized material are contracted or collapsed following evacuation. X-ray powder
diffraction analysis of the CH3CN-exchanged material showed a loss of crystallinity after drying
(see Figure S30, Supporting Information). This outcome is disappointing for a 3-dimensional
framework material; nonetheless, such instability can be rationalized as a consequence of the
15
flexibility of the btp ligand supporting the bulk material, and may also be related to the expected
lability of the pivotal Zn2 node.
Cation exchange experiment
Due to the apparent reduction in free volume of 1 upon evacuation, an alternative investigation
into the guest uptake properties was undertaken, taking advantage of the anionic framework of 1.
It was found that soaking 1 in concentrated solutions of cationic dyes such as ethidium bromide
and methylene blue for 3 days resulted in uptake of the dyes, evident from a color change
observed in the individual crystals (Figure 5).
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Figure 5. Optical microscope photographs of crystals of 1 (A) before cation exchange and after cation exchange with (B) ethidium bromide and (C) methylene blue.
To confirm that this color change was not simply surface adsorption of the dye, the crystals were
soaked with CH3CN solution for several days until the supernatant ran colorless; after this time
no further leeching of dye into the supernatant was observed. The characteristic red color and
blue color of ethidium bromide and methylene blue, respectively, can clearly be seen in the
thinner and more transparent crystals whereas the thicker larger crystals appeared opaque. The
17
CH3CN solvent was then replaced with a concentrated stock solution of tetraethylammonium
iodide (TEAI) in CH3CN. Immediately upon addition, the supernatant took on the color of the
escaping dye molecules as they were exchanged with the TEA cations. Specific aliquots of the
supernatant solution were taken at defined time intervals and the indicative absorbance recorded.
In each case, the dye-loaded samples showed a release of the respective dye molecule into the
supernatant. The absorbance plot and release profile for methylene blue are shown in Figure 6.
Figure 6. Changes in UV-Vis spectrum of supernatent of 1 (6 mg) previously soaked in a concentrated
solution of methylene blue in CH3CN upon addition of triethylammonium iodide in CH3CN (c = 16.4
mM, 4 mL) solution at 21oC.
Bands at 204 nm and 247 nm did not undergo any significant changes indicative of the excess
TEAI in the solution mixture, while the bands at 293 nm and 655 nm increased in absorbance as
the methylene blue guest molecules were liberated from the solid 1. The concentration increased
substantially until between 90 and 120 minutes after the TEAI solution was added and at this
point no further increase was observed. The changes in the absorption spectrum could be
quantified using the measured extinction coefficient of methylene blue in CH3CN, measured in
16.4 mM solution of TEAI in CH3CN as (UV-Vis (MeCN) λmax / nm (εmax /dm3 mol-1 cm-1): 291
18
(37,000) 655 (77,000)). The amount of methylene blue dye released from the first TEAI soaking
corresponds to 23% of the total cationic species required for charge balance within the
framework. Further release of dye was observed when a fresh solution of TEAI (3 mL) was
added to the same sample of 1 after removal of the original supernatant (Figure S17, Supporting
Information) and the release was at its maximum between 100 and 150 minutes after which no
further substantial increase was observed. It was calculated from this that a further 8% of dye
had been exchanged. A third replacement of the supernatant with fresh solution of TEAI in
CH3CN gave rise to a further release of 4% of dye (Figure S18, Supporting Information). In each
case, the blue color of the crystals remained evident following soaking, indicating that complete
removal of the dye guest was not forthcoming under these conditions. The same cation exchange
was carried out with ethidium bromide solution in CH3CN (Figure S19, Supporting Information).
Approximately 9% of the total cationic species, in the form of the ethidium cation, was released
when TEAI solution was added. The samples were then digested in deuterated TFA; the 1H
resonance 2.91 ppm indicative of the dimethylammonium cation was not observed in the dye-
soaked, TEAI-exchanged samples (Figure S14, Supporting Information). The small amount of
methylene blue and ethidium bromide still present in the samples accounted for the release of
blue and red color, respectively, into solution upon digestion of the crystals in TFA-d1. In
particular, the residual methylene blue was evidenced by the diagnostic methyl resonance at 3.59
ppm (Figure S15, Supporting Information) which integrated to 0.4 protons per L, equating to
<10% of the total cationic species. This also contributes to the explanation of why the total
amount of methylene blue released by exchange with TEAI solution was observed to be less than
the total possible. However, the most contributory factor to the total release of methylene blue
accounting only for approximately ca. 35% of the amount expected is most likely the less than
19
complete exchange of the dimethylammonium cation. However, the remaining
dimethylammonium was efficiently replaced by the TEA during the second cation exchange
step. Integration of the 1H NMR resonance indicative of TEA cation at 3.29 ppm and 1.38 ppm
show the exchange with methylene blue and any original dimethylammonium was almost
quantitative i.e. 6:4 (L:TEA) ratio evidenced by peaks e,1 and 2 integrating in a 1: 1.33: 2 ratio
(Figure S15, Supporting Information).
The smaller volume of the TEA cation, and higher concentration of the stock solution used
relative to methylene blue, most likely accounts for the more favorable uptake by 1. For this
same reason, methylene blue was less efficient compared to TEA in the replacement of the
dimethylammonium cation initially, but more favorable than the substantially larger ethidium
cation. Although complete exchange of the lattice cations in 1 with larger cations was not
observed, the uptake of up to 35% of methylene blue, and complete exchange for
tetraethylammonium, is an encouraging observation for the use of 1 as a cation exchange
material, and implies the retention of a considerable solvent-accessible volume.
Spectroscopic monitoring of Ln(III) exchange and sensitization studies
It is well known that the btp terdentate motif is capable of sensitizing Ln(III) luminescent states
11e,15a,16,19c with some reports achieving photoluminescence quantum yields of 70% in CH3CN
solution.34 With this in mind, and encouraged by the ability of 1 to readily undergo cation
exchange, a sample of 1 which had been previously soaked in CH3CN was treated initially with
Eu(CF3SO3)3·6H2O solution in CH3CN, containing 0.16 eq. of Eu(III) w.r.t. 1. Sensitization of
the Eu(III) by 1 occurred immediately, evidenced by the characteristic red phosphorescence
emitted from the crystals of 1 under UV radiation at 365 nm, which was clearly visible to the
naked eye as demonstrated in Figure 7. It should be noted that prior to exchange with Ln(III) ion,
20
the as-synthesized 1 did not display any measurable luminescent whatsoever, despite using a
range of UV excitation wavelengths. The phosphorescence changes were also monitored at
specific time intervals (Figure 7).
Figure 7. (Left) Eu(III) centered phosphorescence observed increasing with time after compound
1 (11 mg, 3.49 µmol) was treated with Eu(CF3SO3)3·6H2O in CH3CN solution (0.58 mM, 1 mL,
0.16 eq.) and. (Right) Crystals of 1 under 365 nm UV radiation and in daylight.
Eu(III) centered phosphorescence intensity plateaued when 1 was treated with dilute
Eu(CF3SO3)3·6H2O in CH3CN (1mL, 0.16 eq., 0.32 eq. w.r.t. 1). Upon addition of more
concentrated solutions (1 mL, 3.5 eq. w.r.t. 1) further increases in phosphorescence intensity
were observed over time. It should be noted that the absolute phosphorescence intensities could
not be quantitatively determined as a function of time due to the unpredictable scattering and
packing effects of the solid materials. Nonetheless, we observed an unmistakable qualitative
increase in overall phosphorescence intensity (further details of the Eu(III) sensitization
experiment can be found in the Supporting Information). The lanthanide emission was shown to
be rapidly quenched upon addition of TEAI in CH3CN solution (6.39 mM, 1 mL) (Figure S27,
21
Supporting Information). This is as a result of the rapid displacement of the cationic Eu(III) ions
by the TEA cations.
The same experiment was carried out with Tb(III) by treating a sample of 1 which had been
previously soaked in CH3CN with a Tb(CF3SO3)3·6H2O solution in CH3CN (0.16 eq. w.r.t. 1).
Immediately, sensitization of the Tb(III) occurred evidenced by the characteristic green
phosphorescence emitted from the crystals of 1 under UV radiation at 365 nm, as shown in
Figure 8.
Figure 8. Tb(III) centred emission observed after treatment of 1 with a solution of
Tb(CF3SO3)·6H2O in CH3CN (3.5 eq. w.r.t. 1) increasing with time. (Inset) Crystals of 1 under
365 nm UV radiation.
As before the Tb(III) emission changes were monitored at specific time intervals showing an
increase in Tb(III) centered phosphorescence over time. To begin, a dilute solution of Tb(III)
was added (1mL, 0.16 eq. w.r.t. 1), and as in the case of the Eu(III), upon addition of more
concentrated solutions (3.5 eq. w.r.t. 1) further increases in the phosphorescence intensity were
observed over time (further details of the Tb(III) sensitization experiment can be found in the
22
Supporting Information, including blending the red and green colors emitting from 1). It was also
investigated if there was any leeching of ligand from the extended structure of 1 by carrying out
UV-Vis spectra of the supernatants of each sample. There was no indication of btp absorbtion
bands observed, Figure S25(left). It was also concluded that the metal centred phosphorescence
was produced exclusively by the crystals after it was confirmed there was no phosphorescence
observed from the supernatants when crystals of 1-Eu and 1-Tb were not in the beampath, Figure
S25(right).
Radiative lifetime studies were also carried out displaying biexponential radiative decay curves
indicative of two Eu(III) environments and two Tb(III) environments in 1-Eu and 1-Tb,
respectively. The observed radiative lifetimes had values of on average τ1 = 0.48(01) ms and τ2 =
1.91(10) ms for the Eu(III) species and of on average τ1 = 0.92(01) ms and τ2 = 4.45(14) ms for
the Tb(III) species. A summary of the observed lifetimes is presented in Table S1 (Supporting
Information).
Conclusion
A three dimensional anionic coordination polymer, 1, with formula [Zn7L6]·(H2NMe2)4·(H2O)45
derived from a semi-flexible tri-carboxylic acid btp ligand was synthesized in good yield with
excellent bulk purity achieved. A second crystalline phase, 2, which was present as a byproduct
in initial attempts was successfully removed through synthetic optimization. The extended
structure of 1 was characterized using conventional methods: single crystal and powder X-ray
diffraction analysis, gas adsorption studies together with thermogravimetric and elemental
analysis. A crystal structure of the btp ligand, H3L, was also obtained, displaying a three-
dimensional hydrogen bonding network. While a reduction in void volume was observed
following the complete evacuation of 1, efficient guest exchange was observed in solution, using
23
cationic dye molecules as probes. Uptake and sensitization of Eu(III) and Tb(III) within the
framework of 1 was also achieved. These observations demonstrate the potential of materials
exhibiting poor structural resilience to complete evacuation such as 1 as nonetheless useful
cation exchange matrices for the development of functional luminescent materials. Efforts are
currently being made to appropriately tune the uptake of various luminescent guests by 1 in the
pursuit of customizable White Light Emitting materials.
Experimental
Materials and methods
All solvents and chemicals were purchased from commercial sources and used without further
purification.
Melting points were determined using an Electrothermal IA9000 digital melting point
apparatus and are uncorrected. Infrared spectra were recorded on a Perkin Elmer Spectrum One
FT-IR spectrometer equipped with universal ATR sampling accessory. Thermogravimetric
analysis was performed on a Perkin Elmer Pyrus 1 TGA equipped with an ultra-micro balance
with a sensitivity of 0.1 microgram. The temperature range is from 25-500 °C with a scan rate 5
°C/min (Figure S6/7). Emission (fluorescence, phosphorescence and excitation) spectra and
lifetimes were recorded on a Varian Cary Eclipse Fluorimeter at at 298 K. Phosphorescence data
were collected between 570 nm and 720 nm for Eu(III) emission and between 565 nm and 575
nm for Tb(III) emission with a measurement delay time of 0.1 ms. Phosphorescence lifetimes of
the Ln(III) excited states were measured in time-resolved mode at 298 K. Elemental analysis was
carried out on Exeter Analytical CE440 elemental analyser at the microanalysis laboratory,
School of Chemistry and Chemical Biology, University College Dublin. Phase purity of all
24
crystalline materials was confirmed with X-ray powder diffraction patterns recorded with a
Bruker D2 Phaser instrument using Cu Kα (λ = 1.5405 Å) radiation. Samples were finely ground
and applied to a quartz sample holder. Data were measured at room temperature in the 2θ range 5
- 55° in 0.01° increments with concurrent rotation in φ of 1 RPM. Additional X-ray powder
diffraction data for 1 was collected at 100K on a Bruker Apex-II Duo instrument using Cu Kα
radiation in the 2θ range 2 - 55°, and was converted to θ vs intensity data by integration of Debye
rings obtained from the area detector data. Raw data were compared with the simulated patterns
from the single crystal data collections carried out at 100 K (Figure S28, S29, S33). Gas
adsorption isotherms were measured using a Quantachrome Autosorb IQ gas sorption analyser.
Chemically pure (CP, N4.5) grade He, N2, H2 and CO2 gases were used for the measurements.
Crystallography
Single crystal X-ray data for H3L were collected at 100 K on a Rigaku AFC12 goniometer
equipped with an enhanced sensitivity (HG) Saturn 724+ detector mounted at the window of an
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Graphical Abstract For Table of Contents only
SynopsisStructural analysis of a three dimensional metal-organic framework material, derived from a flexible tri-carboxylic acid 2,6-bis(1,2,3-triazol-4-yl)pyridine ligand, capable of rapid and reversible guest exchange of various cations and sensitization of lanthanide ions.